So many things had to happen for us (humans) to end up here (Earth). Our species is made possible because a water-filled planet orbits a particular distance away from a star of a particular size. Before we could evolve, living cells, animals, and primates needed to come first. A fish needed to walk onto land. Considering the many necessary pre-conditions and precursors along our winding path, it can feel accidental, bordering on miraculous, that we exist at all.
For decades, the “hard-steps” model of humanity’s origins has bolstered this idea, suggesting that–given how long it took us to evolve relative to the total timeline of the Sun and Earth–our emergence was deeply improbable. According to the hard-steps model, that extreme implausibility holds true across the universe: The evolution of any human-like intelligent life would be far-fetched on any planet.
But a new counter theory upends that idea. Intelligent life on Earth and beyond may be much more commonplace than we’d previously thought, according to a paper published February 14 in the journal Science Advances. The qualitative review study offers a detailed critique of the hard-steps model and presents an alternative way of understanding why it took billions of years for our species to evolve. If we were to go extinct, some other form of intelligent life could readily emerge in our stead, according to the newly proposed framework. And humanity is less likely to be alone in the universe than we thought. Though not direct proof of aliens, the theory offers a way forward for testing and studying where, when, and if aliens might exist.
“Our existence is probably not an evolutionary fluke,” says Jennifer Macalady, a study co-author and microbiology professor at Pennsylvania State University. “We’re an expected or predictable outcome of our planet’s evolution, just as any other intelligent life out there will be.”
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